Conspiratorial old Central American is having a laugh? (2,7)
I believe the answer is:
in cahoots
'conspiratorial' is the definition.
(thesaurus)
'old central american is having a laugh?' is the wordplay.
'old central american' becomes 'incas' (I can't explain this - if you can you should believe this answer much more).
'is having' means one lot of letters goes inside another.
'a laugh?' becomes 'hoot' (I've seen this before).
'incas' enclosing 'hoot' is 'IN CAHOOTS'.
Can you help me to learn more?
(Other definitions for in cahoots that I've seen before include "Thick" , "Conniving" , "Working with" , "Associated with" , "Conspiring together" .)